Numerous methods for adding flavorants to tobacco smoke are known in the art. None of these methods, however, has proven completely satisfactory, particularly where the desired flavorant is menthol.
The addition of menthol per se to tobacco has been found to be ineffective. The highly volatile nature of this compound results in a relatively high loss factor incident to the storage and manufacturing stages required for the production of a smoking product. Quite clearly, such loss is undesirable from an economic standpoint.
In an attempt to alleviate these problems, it has been suggested that menthol might be adsorbed on a suitable support, such as activated charcoal or fuller's earth, and that the resultant composition might then be added to the tobacco. Attempts to pursue this method have not, however, met with complete success. The menthol yields from such adsorbants have been found to be very low. Moreover, this method obviously necessitates incorporation of the adsorbant into the tobacco, and such a foreign material may result in an undesirable appearance as well as give rise to uneven burning of the tobacco.
In order to overcome these difficulties, it has been suggested that the menthol could be incorporated into the tobacco as a part of a compound -- i.e. a menthol release agent -- in such form that upon burning of the tobacco, the compound would be decomposed to yield the desired menthol flavorant. While considerably more satisfactory than earlier attempts, even this technique has evidenced certain drawbacks.
Bavley et al, U.S. Pat. No. 3,312,226, describes a process whereby menthol is incorporated into tobacco as the carbonate ester of various alcohols, particularly one such as linalool, which were themselves useful flavorants. Upon pyrolysis of these carbonate esters, incident to the normal burning temperatures of the tobacco, the menthol is released to flavor the smoke.
Unfortunately, these simple carbonate esters have not proven wholly satisfactory. They retain one of the difficulties of menthol itself, in that they are somewhat susceptible to migration in the tobacco, and thereby prevent the strict control of quantitative release of menthol to the tobacco smoke during burning. Additionally, the second alcohol of the carbonate ester can prove susceptible to chemical alteration during pyrolysis, thus giving rise to undesirable chemical fragments which may add a chemical aftertaste to the smoke.
The Mold et al U.S. Pat. Nos. 3,332,428 and 3,419,543, offer a slightly different approach to the problem of adding menthol flavor to a tobacco smoke. These patents, like that of Bavley et al, rely upon the formation of a carbonate ester to bind the menthol in a release agent. Here, however, a polyhydroxy compound such as a monosaccharide, disaccharide, trisaccharide, polysaccharide, or glycol is used to fix the menthol in the tobacco. Again there are certain drawbacks.
Because the alcohol linkages of these saccharides and glycols are only primary or secondary in character, the efficiency with which the menthol can be regenerated upon pyrolysis is limited, owing to the opportunity for dehydration of the menthol moiety. Additionally, where attempts were made to utilize menthol-release agents of high menthol proportion -- i.e., agents which would release a high proportion of menthol per unit weight -- it was discovered that menthenes were often produced in addition to menthol elimination, thereby resulting in a bitter tasting tobacco smoke.